The present application relates to a charger for secondary batteries, and a method of charging.
There has been known a method of charging secondary batteries based on combination of constant-current charging and constant-voltage charging. FIG. 1 explains the method of charging, in which the abscissa axis expresses charge current (A), and the ordinate axis expresses battery voltage (V). Region (a-b) corresponds to the range of constant-voltage charging, and region (c-d) corresponds to the range of constant-current charging. A power source unit for charging performs constant-voltage regulation in the region (a-b), and performs constant-current regulation in the region (c-d).
For example, in the region where a battery voltage Vb is 4.1 V or below, constant-current charging is performed by a charge current Ib being 500 mA. When the battery voltage exceeds 4.1 V, the power source unit performs constant-voltage regulation to gradually lower the charge current Ib. Battery voltage Vb rises towards an output voltage Vo of the power source unit of 4.2 V, and the charging comes to the end.
An example of related art charger is shown in FIG. 2. The configuration shown in FIG. 2 is aimed at explaining both of current detection and ΔV detection, as methods of detecting the state of charging, and provision of either one of which will be sufficient. A power source unit 1 is connected to a commercial power source with an AC connector CN11 in between, and outputs an output voltage Vo of, for example, 4.2 V to output terminals 1a and 1b. Output voltage of the power source unit 1 is supplied to a secondary battery 2 with a detection/charging unit 3 in between, and thereby the secondary battery 2 is charged.
The detection/charging unit 3 is controlled by a charge control unit 4. The charger includes the detection/charging unit 3 and the charge control unit 4. The charge control unit 4 controls charging and discharging of the secondary battery 2, and is typically configured as an IC circuit containing a microcomputer. Although not shown in the drawing, positive output voltage of the power source unit 1 is supplied to a regulator and is adjusted therein to a predetermined voltage, and the predetermined voltage is supplied to the charge control unit 4 as a source voltage. Between the positive output terminal 1a of the power source unit 1 and the positive terminal of the secondary battery 2, a switching element S11 and a diode D11 having a forward polarity with respect to charge current Ib are connected in series. The switching element S11 is controlled by the charge control unit 4. The charge current Ib is not supplied, when the switching element S11 is switched OFF.
Between the negative output terminal 1b of the power source unit 1 and the negative terminal of the secondary battery 2, a resistor Rx for current detection is connected in series. A series connection of resistors R11 and R12 is connected in parallel with the power source unit 1 between the output terminals 1a and 1b thereof. A series connection of resistors R13 and R14 is connected, at the cathode side of the diode D11, in parallel with the secondary battery 2 between the positive and negative terminals thereof.
Voltage Vx extracted from the middle point of connection between the resistors R11 and R12, and voltage Vy extracted from the middle point of connection between the resistors R13 and R14 are supplied to the charge control unit 4. On both sides of the resistor Rx, voltage Ex corresponding to the charge current Ib is generated. The voltage Ex is supplied to an operation amplifier OP11, and compared with detection voltage Ei generated by a voltage source IC1. Output voltage of the operation amplifier OP11 is supplied to the charge control unit 4.
The current detection system uses a factor that the charge current Ib decreases when the constant-voltage is regulated at the final stage of charging. In the system, the charge current Ib is converted to voltage Ex by the resistor Rx, and the voltage Ex is compared with the detection voltage Ei to determine the state of charging. The system is effective for devices having relatively small charge current Ib, because the resistor Rx causes a loss ascribable to the charge current Ib. The ΔV detection system measures output voltage Vo of the power source unit 1 and battery voltage Vb of the secondary battery 2 connected to the detection/charging unit 3, and determines the state of charging based on difference ΔV therebetween. The ΔV system is effective for devices having relatively large charge current Ib, because the state of charging is determined by voltage measurement. An exemplary charger based on the ΔV system is described in Japanese Patent Application Publication No. JP H6-014473 (Patent Document 1).
Accuracy of the charger configured as shown in FIG. 2 will be explained. First, accuracy of the current detection system will be explained.
Assuming a detection resistor as Rx, detection current as Ix, and offset voltage presented between the positive and negative output terminals of the operation amplifier as Vio, the detection voltage Ex isEx=Ix×Rx  (1)
When the detection voltage Ei equals to Ex, measurement error Q ascribable to the offset voltage isQ=Vio÷Ei  (2)
The detection accuracy may be improved by using high precision components respectively for the operation amplifier OP11, the voltage source IC1 and the detection resistor Rx, but costs for the components will increase. The detection accuracy may be improved also by raising the detection voltage Ei, but due to loss P of the resistor Rx expressed asP=Ib×Ib×Rx  (3)
the efficiency will degrade, and a high-power-type products will be necessary, thereby the cost increases, and the downsizing of components becomes difficult.
The detection accuracy in the ΔV system will be explained next, referring to FIG. 1. The charge control unit 4 generally includes an one-chip microcomputer or the like, and an operation voltage of the unit is lower than the output voltage Vo of the power source unit 1, therefore the detection voltage input into the unit must be lowered by resistive division.
The detection voltage Vx extracted from the middle point between the resistors R11 and R12 is written.Vx=Vo×R12÷(R11+R12)  (4)
Similarly, the voltage Vy extracted from the middle point between the resistors R13 and R14 is writtenVy=Vb×R14÷(R13+R14)  (5).
If defined asR11÷R12=A 
then the equation (4) givesVx=Vo÷(A+1)  (6)
By expressing accuracy of the resistor R11 as G, and accuracy of the resistor R12 as H, a condition Bh making Vx large under the same Vo is writtenBh=(1−G)÷(1+H)  (7)
and a condition Bs making Vx small is writtenBs=(1+G)÷(1−H)  (8)
By expressing Vx taking the resistor errors into consideration as Vxh and Vxs, respectively, they are expressedVxh=Vo÷(A×Bh+1)  (9)Vxs=Vo÷(A×Bs+1)  (10)
As a consequence, detection error Q affected by the accuracy of the resistance value of resistive division is writtenQh=(Vxh−Vx)÷Vx  (11)Qs=(Vxs−Vx)÷Vx  (12)
in which Qh≈Qs holds if G=H.
Assuming now (R11=R13, R12=R14: condition 1), Vy will have an error similarly to Vx. Given, for example, as Vo=10 V, R11 to R14=10 kΩ±0.5%, Vx and Vy will respectively have an error of ±0.5%. Resistors having an accuracy of ±0.5% are understood as high-precision components. This means that approximately 1% of Vo is undetectable due to resistivity errors, when ΔV(=Vo−Vb) is detected. If Vo=10 V, 100 mV is undetectable.
In general, detection of the charge state must be sensible to a ΔV of several millivolts to several tens millivolts. In other words, an undetectable voltage of 100 mV raises a need of some countermeasure. The accuracy may be improved by setting the resistor R14 a variable resistor and adjusting it, but adjustment error will remain, thereby causing increase in the cost and inhibiting the downsizing. Another issue is that the secondary battery 2 after completion of charging is discharged due to the resistors R13, R14 connected thereto, thereby causing lowering in the battery capacity, and degradation of the battery induced by re-charging.
Still another issue is that, when operation of the power source unit 1 is stopped while the secondary battery 2 is connected thereto, the battery voltage is applied through the resistor R13 to the charge control unit 4 to cause a state of reverse voltage, even though the operation source voltage to the charge control unit 4 is 0 V, thus thereby raising a need of protection for IC and so forth composing the charge control unit 4. On the other hand, in order to prevent the charge control unit 4 from being applied with reverse voltage when the output voltage Vo of the power source unit 1 is brought into an over-voltage state, values of Vx must be equal to or smaller than the operation source voltage of the charge control unit 4. In addition, a small quotient (Vx÷Vo), or a large ratio of Vo to Vx, will degrade the detection accuracy, because the amount of change of Vx relative to the amount of change of Vo becomes small.
A description of Patent Document 1 in FIG. 1, FIG. 7 and FIG. 13 is provided below.
In FIG. 1, charge output voltage Vo1 is measured during charging at a position nearer to the secondary battery 9 side than the switch unit 8. This means that result of measurement is modified, because measurement of Vo1 contains voltage drop ascribable to the charge current during the measurement and impedance of the switch unit 8.
In FIG. 7, the charge current is cut off at certain intervals, and when voltage difference between the output voltage VA1 at the power source side and voltage VA2 at the battery side in the cut off state is measured, voltages at a plurality of points are measured with a single detection circuit by selecting connection points by a switch. This may successfully exclude measurement errors among the detection circuits possibly occur when the detection circuits are provided to the individual points of measurement, but increase the cost caused by a component for the switching. This problem is similar as illustrated in FIG. 13 of Patent Document 1.